In the late 1980’s Sir Sam Edwards proposed a framework for describing the large scale properties of granular materials, such as sand or salt. In this description, similar to the well-established framework of statistical mechanics, the global properties of a complex system are determined by an average over all possible microscopic configurations consistent with a given global property. This is usually attributable to the very fast dynamics of the constituent particles making up the system. The extension of such treatments to granular systems where particles are static or ‘jammed’ represents a fundamental challenge in this field. Even so, Edwards’ conjecture postulated that for given external parameters such as volume, all possible packings of a granular material are equally likely. Such a conjecture, like Boltzmann’s hypothesis in statistical mechanics, can then be used as a starting point to develop new physical theories for such materials based on statistical principles. Indeed, several frameworks have been developed assuming this conjecture to be true.

Figure 1 : Snapshot of the system studied and illustration of the associated energy landscape at different volume fractions.

A simple illustration of this conjecture would be, if one were to pour sand into a bowl, and not bias the preparation in any way, then all the trillion trillions of configurations allowed for the grains would be equally likely. Clearly such a conjecture is utterly infeasible to test experimentally. In a recent paper that appeared in Nature Physics, we instead performed detailed numerical computations on a theoretical system of soft disks (in two dimensions) with hard internal cores. We focused on a system of 64 disks which already pushed the limits of current computational power. We found that if one fixes the density of a given system of disks, the probability of a packing occurring depends on the pressure, violating Edwards’ proposition. However, at a critical density, where particles just begin to touch or ‘jam’, this probability remarkably becomes independent of the pressure, and all configurations are indeed equally likely to occur. This jamming point is in fact very interesting in its own right since most granular materials are found at the threshold of being jammed and ‘unjammed’. To be fair to Edwards, the hypothesis was made for ‘hard’ grains in which particles are precisely at this threshold, and therefore our numerics seem to confirm the original statement. This is the first time that this statement has been out to a direct test and will no doubt lead to many interesting directions in the future.

Pumping water through a pipe solves the need to provide water in every house. By turning on faucets, we retrieve water at home without needing to carry it from a reservoir with buckets. However, driving water through a pipe requires external pressure; such pressure increases linearly with pipe length. Longer pipes need to be more rigid for sustaining proportionally-increased pressure, preventing pipes from exploding. Hence, transporting fluids through pipes has been a challenging problem for physics as well as engineering communities.

To overcome such a problem, Postdoctoral Associate Kun-Ta Wu and colleagues from the Dogic and Fraden labs, and Brandeis MRSEC doped water with 0.1% v/v active matter. The active matter mainly consisted of kinesin-driven microtubules. These microtubules were extracted from cow brain tissues. In cells, microtubules play an important role in cell activity, such as cell division and nutrient transport. The activity originates from kinesin molecular motors walking along microtubules. In cargo transport, microtubules are like rail tracks; kinesin motors are like trains. When these tracks and trains are doped in water, their motion drives surrounding fluids, generating vortices. The vortices only circulate locally; there is no global net flow.

Figure: Increasing the height of the annulus induces a transition from locally turbulent to globally coherent flows of a confined active isotropic fluid. The left and right half-plane of each annulus illustrate the instantaneous and time-averaged flow and vorticity map of the self-organized flows. The transition to coherent flows is an intrinsically 3D phenomenon that is controlled by the aspect ratio of the channel cross section and vanishes for channels that are either too shallow or too thin. Adapted from Wu et al. Science 355, eaal1979 (2017).

Back in the summer of 2015, Agon had the opportunity to visit KITP during two important programs on the physics frontiers, both of special interest to him, namely ”Entanglement in Strongly-Correlated Quantum Matter” and ”Quantum Gravity Foundations: UV to IR”. That was a great opportunity to meet in person the leaders of the field from around the world in the relaxed and friendly atmosphere of the KITP. Discussions among the researchers and students were tremendously common all around the institute and there were many activities that facilitated such discussions such as daily coffees, lunches, and dinners.

W. Benjamin (Ben) Rogers is currently a research associate in Applied Physics at Harvard University under the supervision of Professor Vinothan Manoharan. Before coming to Harvard, he completed his Ph.D. in the Department of Chemical and Biomolecular Engineering at the University of Pennsylvania and his B.S. in Chemical Engineering from the University of Delaware.

Ben’s research focuses on developing quantitative tools and design strategies to understand and control the self-assembly of soft matter. He is interested in elucidating the role of specificity in complex self-assembly, designing responsive nanoscale materials by controlling phase transitions in colloidal suspensions, and understanding how coupled chemical reactions give rise to active materials, which can move, organize, repair, or replicate. At the intersection of soft condensed matter, biophysics, and DNA nanotechnology, his research utilizes techniques from synthetic chemistry, optical microscopy, micromanipulation, and statistical mechanics.

Jané has advised first year students and majors, served on senior thesis and dissertation committees, and supervised undergrads, grads and post-docs working in his lab. Additionally, he had chaired the Physics department, served as chair and Undergraduate Advising Head of the Biological Physics program, and co-directed the Quantitative Biology graduate program. His courses include the first year seminar, “Nature’s Nanotechnology,” as well as “Advanced Introductory Physics,” “Biological Physics” and “Quantum Mechanics.”

Jané earned his BS at the University of Belgrade and his PhD at Cornell University, and a postdoc at Brown University, where he won two Excellence in Teaching Awards. The goal of his research at Brandeis is to develop quantitative models of biological structure and function that can be tested experimentally. His current projects include the study of cell-to cell variability in gene expression, homologous recombination in yeast, synthetic genetic circuits, and formin assisted actin assembly.

Jané’s research has been supported by grants from the National Institutes of Health, the National Science Foundation and the MIT Whitehead Institute. His co-authored undergraduate textbook, Physical Biology of the Cell, won the 2013 Society of Biology Book Award, and his articles have been published in such journals as the Physics Today, Genetics, Cell Reports, and Biophysics.

Students in his courses write:

“Jané is an awesome instructor. He really cares that the students understand the material.”

“I learned a lot from informal conversations with Professor Kondev, and I appreciate all the energy and passion that he brings to the classroom.”